Summary Advertisements

In 5 minute intervals, a VTP server will send a Summary Advertisement to it’s adjacent switches. Included in this VTP Summary Advertisement is the VTP domain name and configuration revision number.

Another VTP Server or Client receives the Summary Advertisement and compares the VTP domain name and revision number to its own. If the domain name is different, it ignores the advertisement. If its own configuration revision number is higher than the request, it ignores the advertisement. If the configuration revision number is higher than its own, it sends an advertisement request. Following Summary Advertisements are Subset Advertisements.

Subset Advertisements

Changes made to VLANs will increment the configuration revision number and issue a Summary Advertisement followed by one or more subset advertisements. The Subset Advertisement is what holds the VLAN information.

Advertisement Requests

When a switch needs VTP information it sends an Advertisement Request. This request is sent if the switch has been reset, VTP domain name has changed, or it received a VTP Summary Advertisement with a higher configuration revision. Following the Summary Advertisement are Subset Advertisements.

A routed port on a switch can act like a port on a router. Routed switch ports aren’t joined to any VLANs and they do not support VLAN subinterfaces. Because they are routed ports, you can configure a Layer 3 protocol. And because the routed port is acting as a Layer 3 interface it will not support Layer 2 protocols.

Configuring Routed Ports

Get into interface configuration mode of a switch and issue the command no switchport. This disables Layer 2 capabilities and enables Layer 3. You can then assign an IP address to the interface.

Although limited, you can enable a routing protocol to assign routing capabilities to the port.

Back when I was getting started in IT, Spanning Tree Protocol was a lightly touched upon topic. “Just remember it’s on by default and it does most of the work for you.” Most small networks won’t have to configure STP. If you’re planning redundant network links then you have to pay attention to STP.

STP starts with 802.1D. It provides a loop-free topology on a Layer 2 network. When a failure occurs, the network can recover without the intervention from an administrator. A switch with STP will look for redundant links to other switches. When redundant links are found, the switch blocks one of those links to prevent a loop from occurring.

A switch listens on incoming frames to find out what network devices reside on which ports. A table is built with source MAC addresses with the port numbers where the switch found other network devices.

When switches aren’t aware of each other, a bridging loop can occur. This is when a frame is forwarded continuously between switches.

BPDUs

Switches communicate with each other via BPDUs, or bridge protocol data units. They are sent out every 2 seconds from the switch’s MAC address to a multicast address of 01:80:C2:00:00:00.

Two types of BPDUs can be transmitted:

Configuration BPDU – for STP computation.

Topology Change Notification (TCN) – sent when changes occur in the network topology.

VLAN Trunk Protocol (VTP) is a Cisco-proprietary protocol used to replicate VLANs using Layer 2 trunk frames to other switches. It is a method to ease administration by selecting a switch to be configured as a VTP server.

VLANs are configured on the VTP server and then replicated to other connected switches, called VTP clients. This reduces the amount of configuration — having to log onto each switch to create a VLAN. All VLAN and VTP data is saved in the vlan.dat file – stored in the switch’s flash memory.

VTP Domain

VTP is organized into management domains, which share common VLAN requirements. These management domains are called VTP Domains. A switch can only be joined to one VTP domain. Each VTP domain is isolated from one another. Information is not shared between VTP domains.

VTP Modes

Switches can be configured in one of three VTP modes:

Server Mode – switch has full control over creating and modifying VLANs. This is the default mode for a switch.